Amines and Related Nitrogen Compounds - Cengage Learning - page 211

11
Amines and Related Nitrogen Compounds
Chapter Summary
Amines are organic derivatives of ammonia. They may be primary , secondary , or tertiary ,
depending on whether one, two, or three organic groups are attached to the nitrogen. The
nitrogen is sp 3 -hybridized and pyramidal, nearly tetrahedral.
The amino group is -NH 2 . Amines are named according to the Chemical Abstracts
(CA) system by adding the suffix - amine to the names of the alkyl groups attached to the
nitrogen. Amines can also be named using the IUPAC system in which the amino group is
named as a substituent. Aromatic amines are named as derivatives of aniline or of the
aromatic ring system.
Primary and secondary amines form intermolecular N-H···N bonds. Their boiling
points are higher than those of alkanes but lower than those of alcohols with comparable
molecular weights. Lower members of the series are water-soluble because of N···H-O
bonding.
Amines can be prepared by S N 2 alkylation of ammonia or 1 o and 2 o amines. Aromatic
amines are made by reduction of the corresponding nitro compounds. Amides, nitriles, and
imines can also be reduced to amines.
Amines are weak bases. Alkylamines and ammonia are of comparable basicity, but
aromatic amines are much weaker as a result of delocalization of the unshared electron pair
on nitrogen to the ortho and para carbons of the aromatic ring. Amides are much weaker
bases than amines because of delocalization of the unshared electron pair on nitrogen to
the adjacent carbonyl oxygen. Amides are stronger Brønsted acids than amines because of
the partial positive charge on the amide nitrogen and resonance in the amidate anion .
Amines react with strong acids to form amine salts . The p K a s of amine salts are
related to the base strength of the corresponding amines. Alkylammonium salts have p K a s
of 9-10 while arylammonium salts have p K a s of 4-5. The fact that these salts are usually
water-soluble can be exploited in separating amines from neutral or acidic contaminants.
Chiral amines can be used to resolve enantiomeric acids, through the formation of
diastereomeric salts.
Primary and secondary amines react with acid derivatives to form amides. Amides
made commercially this way include acetanilide and N,N -diethyl- m -toluamide (the insect
repellent Off ® ).
Tertiary amines react with alkyl halides to form quaternary ammonium salts .
An example of this type of salt with important biological properties is choline
(2-hydroxyethyltrimethylammonium ion).